these clauses tell you exactly which person or thing is being discussed (i.e. they identify the noun)

e.g. Parks and reserves which are controlled by the Christchurch City Council are now being considered as potential sites for cell phone towers.

In other words, only some of Christchurch's parks and reserves are controlled by the Christchurch City Council, and only those parks and reserves that are controlled by the CCC are being considered potential sites.

essential information clauses have no commas around them

these clauses cannot be left out of the sentence (without changing the meaning or confusing the reader)

these clauses can use that instead of who or which

e.g. Parks and reserves that are controlled by the Christchurch City Council are now being considered as potential sites for cell phone towers.